Transmitter and method of transmitting payload data, receiver and method of receiving payload data in an ofdm system

ABSTRACT

A transmitter transmits payload data using Orthogonal Frequency Division Multiplexed (OFDM) symbols. The transmitter comprises a frame builder configured to receive the payload data to be transmitted and to receive first signalling data for use in detecting and recovering the payload data at a receiver, and to form the payload data and the first signalling data into frames for transmission, the first signalling data forming a part of the frames with the payload data. A modulator is configured to modulate a first OFDM symbol with the first signalling data and to modulate one or more second OFDM symbols with the payload data. A signature sequence processor provides a signature sequence, a combiner combines the signature sequence with the first OFDM symbol, and a transmission unit transmits the first and second OFDM symbols. The signature sequence provided by the signature sequence processor comprises at least one of a first synchronisation sequence or a second message sequence, the first synchronisation sequence and or the second message sequence being combined by the combiner with the first OFDM symbol. The first synchronisation sequence is provided for a receiver to detect and to recover the first signalling data from the first OFDM symbol and the second message sequence provides message information to the receiver. The message information may be used to convey a specific message to a user such as an emergency warning relating to a natural disaster such as an earthquake or a tsunami warning.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application which claims thebenefit of priority under 35 U.S.C. §120 of U.S. patent application Ser.No. 14/778,909, filed Sep. 21, 2015, which is a National StageApplication based on PCT/GB2014/050954, filed Mar. 26, 2014, and claimspriority to Great Britain patent application no. 1305795.5, filed Mar.28, 2013; the entire contents of each of which are incorporated hereinby reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to transmitters and methods oftransmitting payload data using Orthogonal Frequency DivisionMultiplexed (OFDM) symbols.

BACKGROUND OF THE DISCLOSURE

There are many examples of radio communications systems in which data iscommunicated using Orthogonal Frequency Division Multiplexing (OFDM).Television systems which have been arranged to operate in accordancewith Digital Video Broadcasting (DVB) standards for example, use OFDMfor terrestrial and cable transmissions. OFDM can be generally describedas providing K narrow band sub-carriers (where K is an integer) whichare modulated in parallel, each sub-carrier communicating a modulateddata symbol such as Quadrature Amplitude Modulated (QAM) symbol orQuadrature Phase-shift Keying (QPSK) symbol. The modulation of thesub-carriers is formed in the frequency domain and transformed into thetime domain for transmission. Since the data symbols are communicated inparallel on the sub-carriers, the same modulated symbols may becommunicated on each sub-carrier for an extended period. Thesub-carriers are modulated in parallel contemporaneously, so that incombination the modulated carriers form an OFDM symbol. The OFDM symboltherefore comprises a plurality of sub-carriers each of which has beenmodulated contemporaneously with different modulation symbols. Duringtransmission, a guard interval filled by a cyclic prefix of the OFDMsymbol precedes each OFDM symbol. When present, the guard interval isdimensioned to absorb any echoes of the transmitted signal that mayarise from multipath propagation.

As indicated above, the number of narrowband carriers K in an OFDMsymbol can be varied depending on operational requirements of acommunications system. The guard interval represents overhead and so ispreferably minimized as a fraction of the OFDM symbol duration in orderto increase spectral efficiency. For a given guard interval fraction,the ability to cope with increased multipath propagation whilstmaintaining a given spectral efficiency can be improved by increasingthe number K of sub-carriers thereby increasing the duration of the OFDMsymbol. However, there can also be a reduction in robustness in thesense that it may be more difficult for a receiver to recover datatransmitted using a high number of sub-carriers compared to a smallernumber of sub-carriers, because for a fixed transmission bandwidth,increasing the number of sub-carriers K also means reducing thebandwidth of each sub-carrier. A reduction in the separation betweensub-carriers can make demodulation of the data from the sub-carriersmore difficult for example in the presence of Doppler frequency. That isto say that although a larger number of sub-carriers (high orderoperating mode) can provide a greater spectral efficiency, for somepropagation conditions, a target bit error rate of communicated data mayrequire a higher signal to noise ratio to achieve than required for alower number of sub-carriers.

SUMMARY OF DISCLOSURE

According to an example embodiment there is provided a transmitter fortransmitting payload data using Orthogonal Frequency DivisionMultiplexed (OFDM) symbols. The transmitter comprises a frame builderconfigured to receive the payload data to be transmitted and to receivefirst signalling data for use in detecting and recovering the payloaddata at a receiver, and to form the payload data and the firstsignalling data into frames for transmission, the first signalling dataforming a part of the frames with the payload data. A modulator isconfigured to modulate a first OFDM symbol with the first signallingdata and to modulate one or more second OFDM symbols with the payloaddata. A signature sequence processor provides a signature sequence, acombiner combines the signature sequence with the first OFDM symbol, anda transmission unit transmits the first and second OFDM symbols. Thesignature sequence provided by the signature sequence processor isselected from one of a set of signature sequences, the signaturesequence being combined by the combiner with the first OFDM symbol, sothat a receiver can detect and recover the first signalling data fromthe first OFDM symbol. The signature sequences of the set providemessage information to the receiver. The synchronisation sequence isprovided for a receiver to detect and to recover the first signallingdata from the first OFDM symbol before the one or more second OFDMsymbols. The choice of one of the at least two possible sequences canform a second signalling data which the transmitter can use to convey aparticular message to the receiver. If the number of possible sequencesthat the transmitter can use is N, then the number of possible messagesthat can be conveyed through this second signalling data is log₂(N).

The message information conveyed by this second signalling data may alsobe used to detect and recover the payload. In other examples the secondsignalling, data may be used to convey a specific message to a user suchas an emergency warning relating to a natural disaster such as anearthquake or a tsunami warning.

Embodiments of the present disclosure can provide a transmitter, whichis arranged to transmit payload data using Orthogonal Frequency DivisionMultiplexing (OFDM) symbols. The transmitter comprises a frame builderwhich is adapted to receive the payload data to be transmitted and toreceive first signalling data for use in detecting and recovering thepayload data to be transmitted at a receiver. The frame builder isconfigured to form the payload data and the signalling data into framesfor transmission. The first signalling data may be formed into eachframe and transmitted using a first OFDM symbol and the payload data maybe transmitted using one or more second OFDM symbols in accordance withtransmission parameters, such as a coding rate, a modulation scheme andan operating mode for the number of subcarriers for OFDM symbols. Thefirst OFDM symbol may therefore be different from the second OFDMsymbols. The first OFDM symbols may be configured to form a preamble ineach frame and may be configured to be detected first by a receiver inorder to recover the first signalling data.

Embodiments of the present disclosure can provide an arrangement inwhich a signature sequence is combined with OFDM symbols carrying, forexample, signalling data so that there is an improved likelihood of areceiver being able to detect the OFDM symbols carrying the signallingdata. According to an arrangement in which embodiments of the presentdisclosure find application there is a requirement to provide a“preamble” OFDM symbol in a transmission frame, which carries signallingparameters to indicate, for example, at least some of the communicationsparameters which were used to encode and to modulate payload data ontothe data hearing OFDM symbols whereby after detecting the signallingdata within the first (preamble) OFDM symbol the receiver can recoverthe transmission parameters in order to detect the payload data from thedata bearing OFDM symbols. Furthermore, the signature sequence processoris configured to generate either a first synchronisation sequence or asecond synchronisation sequence, the selection of the secondsynchronisation sequence representing information such as the presenceof an emergency warning message of within the first signalling data orwithin the payload.

In some embodiments, the signature sequence may be designed to bedetected first, with the detection of the preamble OFDM symbol in aframe, at lower signal to noise ratios than the payload data. As such,the message sequence can provide an early warning or public broadcastinformation, which is more widely detectable than the payload data.Furthermore, because the message sequence can be detected before thedetection of the payload data, a receiver can be configured to detectthe message sequence even in a standby state or powered-off state byproviding power to only a part of a receiver, which is configured todetect the message sequence.

Various further aspects and features of the disclosure are defined inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described by way ofexample only with reference to the accompanying drawings wherein likeparts are provided with corresponding reference numerals and in which:

FIG. 1 is a schematic diagram illustrating an arrangement of a broadcasttransmission network;

FIG. 2 is a schematic block diagram illustrating an example transmissionchain for transmitting broadcast data via the transmission network ofFIG. 1;

FIG. 3 is a schematic illustration of OFDM symbols in the time domainwhich include a guard interval;

FIG. 4 is a schematic block of a typical receiver for receiving databroadcast by the broadcast transmission network of FIG. 1 using OFDM;

FIG. 5 is a schematic illustration of a transmission frame fortransmitting broadcast data including payload data and signalling data;

FIG. 6 is a block diagram showing a transmitter for transmittingsignalling data via a signalling or preamble OFDM symbol according toone embodiment;

FIG. 7 is a schematic block diagram of a signature sequence generatoraccording to one embodiment;

FIG. 8 is a graphical plot of bit error rate with respect to signal tonoise ratio in the presence of additive white Gaussian noise for codingrates of one half and one quarter;

FIG. 9 is a graphical plot of bit error rate with respect to a signaturesequence back-off from the power of the modulated signalling data, whichprovides an acceptable performance according to the results of FIG. 8;

FIG. 10a is a schematic representation of OFDM symbols with a guardinterval matched to an expected delay spread produced for a singlefrequency transmission network; FIG. 10b is a schematic representationof OFDM symbols with different numbers of sub-carriers per OFDM symbolwith a guard interval selected as a fixed fraction of the related OFDMsymbol duration; and FIG. 10c is a schematic representation of OFDMsymbols with a different number of sub-carriers per payload data bearingOFDM symbol and a different number of sub-carriers for a signalling OFDMsymbol with guard interval selected to have a duration which is matchedto both the payload and the signalling OFDM symbols;

FIG. 11a is a schematic block diagram of a receiver for detecting andrecovering signalling data from a signalling OFDM symbol according tothe present technique. FIG. 11b is a schematic block diagram of afrequency synchronisation detector which forms part of FIG. 11 a, FIG.11c is a schematic block diagram of a preamble guard interval correlatorwhich forms part of FIG. 11 b, FIG. 11d is an illustrative schematicblock diagram of a further example of a coarse frequency offsetsynchronisation detector which forms part of the receiver of FIG. 11 a,and FIG. 11e is an illustrative schematic block diagram of adifferential encoder which forms part of FIG. 11 d;

FIG. 12 is a schematic block diagram of one example of a preambledetection and decoding processor which forms part of the receiver shownin FIG. 11 a, which detects and removes the signature sequence in thefrequency domain;

FIG. 13 is a schematic block diagram of one example of a preambledetection and decoding processor which forms part of the receiver shownin FIG. 11 a, which detects and removes the signature sequence in thetime domain;

FIG. 14 is a schematic block diagram of an example of a signaturesequence remover which forms part of the preamble detection and decodingprocessor shown in FIG. 13;

FIG. 15a is a schematic block diagram of a matched filter, which ismatched to the signature sequence for which an example generator isshown in FIG. 7, and FIG. 15b is a schematic block diagram of asignature sequence remover forming part of the receiver shown in FIG.14;

FIG. 16a is a graphical representation of a signal formed at the outputof the matched filter; FIG. 16b is an expanded view of the graphicalrepresentation shown in FIG. 16a illustrating components of a channelimpulse response;

FIG. 17 is a schematic block diagram illustrating a circuit fordetecting a coarse frequency offset in the receiver of FIG. 11 a;

FIG. 18 is a graphical plot of the correlation output of the circuitshown in FIG. 17 for a frequency offset of −88/Tu;

FIG. 19 provides a graphical plot of bit error rate with respect tosignal to noise ratio for different code rates with and without asignature sequence added to the signalling OFDM symbol for rate one halfand rate one quarter codes;

FIGS. 20a and 20b provide graphical plots of bit error rate againstsignal to noise ratio for a 0 dB echo channel with two paths asillustrated in FIG. 20c respectively with ideal and actual channelestimation; and

FIG. 21a is a schematic block diagram of parts of the transmitter ofFIG. 6 providing a further example embodiment of the present technique;and FIG. 21b is a table showing example parameters of operation of thetransmitter shown in FIG. 21 a;

FIG. 22 is a schematic block diagram and part operational diagramschematically illustrating a formation of the preamble OFDM symbol bythe transmitter of FIG. 21 a;

FIG. 23 is a schematic block diagram of a receiver for detecting asignature sequence of a received signal, which has been transmitted bythe transmitter of FIG. 21 a;

FIG. 24 is a schematic block diagram of an early warning signal detectoraccording to an embodiment of the present technique; and

FIG. 25 is a schematic block diagram of a signalling decoder of thereceiver shown in FIG. 23 providing an example embodiment of the presenttechnique.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Embodiments of the present disclosure can be arranged to form atransmission network for transmitting signals representing dataincluding video data and audio data so that the transmission networkcan, for example, form a broadcast network for transmitting televisionsignals to television receiving devices. In some examples the devicesfor receiving the audio/video of the television signals may be mobiledevices in which the television signals are received while on the move.In other examples the audio/video data may be received by conventionaltelevision receivers which may be stationary and may be connected to afixed antenna or antennas.

Television receivers may or may not include an integrated display fortelevision images and may be recorder devices including multiple tunersand demodulators. The antenna(s) may be inbuilt to television receiverdevices. The connected or inbuilt antenna(s) may be used to facilitatereception of different signals as well as television signals.Embodiments of the present disclosure are therefore configured tofacilitate the reception of audio/video data representing televisionprograms to different types of devices in different environments.

As will be appreciated, receiving television signals with a mobiledevice while on the move may be more difficult because radio receptionconditions will be considerably different to those of a conventionaltelevision receiver whose input comes from a fixed antenna.

An example illustration of a television broadcast system is shown inFIG. 1. In FIG. 1 broadcast television base stations 1 are shown to beconnected to a broadcast transmitter 2. The broadcast transmitter 2transmits signals from base stations 1 within a coverage area providedby the broadcast network. The television broadcast network shown in FIG.1 operates as a so called single frequency network in which each of thetelevision broadcast base stations 1 transmit the radio signalsconveying audio/video data contemporaneously so that these can bereceived by television receivers 4 as well as mobile devices 6 within acoverage area provided by the broadcast network. For the example shownin FIG. 1 the signals transmitted by the broadcast base stations 1 aretransmitted using Orthogonal Frequency Division Multiplexing (OFDM)which can provide an arrangement for transmitting the same signals fromeach of the broadcast stations 2 which can be combined by a televisionreceiver even if these signals are transmitted from different basestations 1. Provided a spacing of the broadcast base stations 1 is suchthat the propagation time between the signals transmitted by differentbroadcast base stations 1 is less than or does not substantially exceeda guard interval that precedes the transmission of each of the OFDMsymbols then a receiver device 4, 6 can receive the OFDM symbols andrecover data from the OFDM symbols in a way which combines the signalstransmitted from the different broadcast base stations 1. Examples ofstandards for broadcast networks that employ OFDM in this way includeDVB-T, DVB-T2 and ISDB-T.

An example block diagram of a transmitter forming part of the televisionbroadcast base stations 1 for transmitting data from audio/video sourcesis shown in FIG. 2. In FIG. 2 audio/video sources 20 generate theaudio/video data representing television programmes. The audio/videodata is encoded using forward error correction encoding by anencoding/interleaver block 22 which generates forward error correctionencoded data which is then fed to a modulation unit 24 which maps theencoded data onto modulation symbols which are used to modulate OFDMsymbols. Depicted on a separate lower arm, signalling data providingphysical layer signalling for indicating for example the format ofcoding and modulation of the audio/video data is generated by a physicallayer signalling unit 30 and after being encoded by an encoding unit 32,the physical layer signalling data is then modulated by a modulationunit 24 as with the audio/video data.

A frame builder 26 is arranged to form the data to be transmitted withthe physical layer data into a frame for transmission. The frameincludes a time divided section having a preamble in which the physicallayer signalling is transmitted and one or more data transmissionsections which transmit the audio/video data generated by theaudio/video sources 20. A symbol interleaver 34 may interleave the datawhich is formed into symbols for transmission before being modulated byan OFDM symbol builder 36 and an OFDM modulator 38. The OFDM symbolbuilder 36 receives pilot signals which are generated by a pilot andembedded data generator 40 and fed to the OFDM symbol builder 36 fortransmission. An output of the OFDM modulator 38 is passed to a guardinsertion unit 42 which inserts a guard interval and the resultingsignal is fed to a digital to analogue convertor 44 and then to an RFfront end 46 before being transmitted by an antenna 48.

As with a conventional arrangement OFDM is arranged to generate symbolsin the frequency domain in which data symbols to be transmitted aremapped onto sub carriers which are then converted into the time domainusing an inverse Fourier Transform. Thus the data to be transmitted isformed in the frequency domain and transmitted in the time domain. Asshown in FIG. 3 each time domain symbol is generated with a useful partof duration Tu seconds and a guard interval of duration Tg seconds. Theguard interval is generated by copying a part of the useful part of thesymbol in the time domain. By correlating the useful part of the timedomain symbol with the guard interval, a receiver can be arranged todetect the useful part of the OFDM symbol of duration Tu, from whichdata can then be recovered by triggering a Fast Fourier Transform toconvert the time domain symbol samples into the frequency domain. Such areceiver is shown in FIG. 4.

In FIG. 4 a receiver antenna 50 is arranged to detect an RF signal whichis passed via a tuner 52 and converted into a digital signal using ananalogue to digital converter 54 before the guard interval is removed bya guard interval removal unit 56. After detecting the optimum positionfor performing a fast Fourier Transform (FFT) to convert the time domainsamples into the frequency domain, an FFT unit 58 transforms the timedomain samples to form the frequency domain samples which are fed to achannel estimation and correction unit 60. The channel estimation andcorrection unit 60 then estimates the transmission channel for exampleby using pilot sub-carriers which have been embedded into the OFDMsymbols. After excluding the pilot sub-carriers, all the data-hearing,sub-carriers are fed to a symbol de-interleaver 64 which de-interleavesthe sub-carrier symbols. A de-mapper unit 62 then extracts the data bitsfrom the sub-carriers of the OFDM symbol. The data bits are fed to a bitde-interleaver 66, which performs the de-interleaving so that the errorcorrection decoder can correct errors in accordance with a conventionaloperation.

Framing Structure

FIG. 5 shows a schematic of the framing structure according to anexample embodiment of the present technique. FIG. 5 illustratesdifferent physical layer frames, some targeted for mobile receptionwhilst others are targeted for fixed roof-top antenna reception. Thesystem can be expanded in future to incorporate new types of frames, forthe current system, these potential new types of frames are simply knownas future extension frames (FEFs).

One requirement for fixed reception frames is an improved spectralefficiency which may be assured by such features as adopting a higherorder modulation, for example 256 QAM, and higher code rates, forexample greater than half rate, because of relatively benign channelconditions, and a high number of sub-carriers per OFDM symbol (FFT size)such as 32K. This reduces the capacity loss due to the guard intervalfraction. However, a higher number of sub-carriers can make such OFDMsymbols unsuitable for mobile reception because of lower tolerance tohigh Doppler frequency of the received signal. On the other hand, themain requirement for mobile reception frames could be robustness inorder to ensure a high rate of service availability. This can beimproved by adopting such features as a low order modulation for exampleQPSK or BPSK, low code rates, a low number of sub-carriers per OFDMsymbol (FFT size) and a high density scattered pilot pattern etc. A lownumber of sub-carriers for OFDM symbols can be advantageous for mobilereception because a lower number of sub-carriers can provide a widersub-carrier spacing and so more resilience to high Doppler frequency.Furthermore a high density pilot pattern eases channel estimation in thepresence of Doppler.

The framing structure shown in FIG. 5 is therefore characterised byframes which may each include payload data modulated and encoded usingdifferent parameters. This may include for example using different OFDMsymbol types having, different number of sub-carriers per symbol, whichmay be modulated using different modulation schemes, because differentframes may be provided for different types of receivers. However eachframe may include at least one OFDM symbol carrying signalling data,which may have been modulated differently to the one or more OFDMsymbols carrying the payload data. Furthermore the signalling OFDMsymbol may be a different type to the OFDM symbol(s) carrying thepayload data. The signalling data is required to be recovered so thatthe payload data may be de-modulated and decoded.

What Characteristics for the Preamble?

To delimit frame boundaries, a frame preamble symbol such as the P1symbol in DVB-T2 is required. The preamble symbol would carry signallingthat describes how the following frame is built. It is expected that allof the types of receiver mentioned above whether mobile or with a fixedantenna should be able to detect and decode the preamble in order todetermine whether or not they should decode the payload in the followingframe. Desirable characteristics for such a preamble include:

-   1. High Capacity of Signalling; The preamble should have a high    signalling capacity—unlike the P1 preamble in DVB-T2 with capacity    of 7 signalling bits, a preamble more like in DVB-C2 with 100s of    signalling bits is desirable. This suggests that the preamble symbol    should be an OFDM symbol with enough sub-carriers to carry all the    signalling information.-   2. Common Macro-Structure; All frame preambles should have a common    pre-defined macro-structure that is understood by all receiver    types. This means that the preamble symbol should have for example a    constant duration, constant number of sub-carriers and guard    interval for all frame types. This forces a constraint that the    guard interval must be similar in duration to the longest guard    interval that may be used in fixed antenna reception, otherwise when    the network uses this longest guard interval, the preamble symbol    will suffer from excessive inter-symbol interference (ISI) and    perhaps suffer decoding failure.-   3. Low Complexity Detection and Decoding: The preamble symbol    detection and decoding complexity should be low enough to easily    implement in battery powered mobile receivers, so as to make    efficient use of limited stored power. This constrains the maximum    FFT size and maximum FEC block length.-   4. The preamble should be easily detected in the time domain; in    DVB-C2, all OFDM symbols within the frame structure use 4K    subcarrier spacing. This means that the receiver can start with OFDM    symbol time synchronisation followed by frequency domain frame    synchronisation (preamble detection). In an embodiment of the    present disclosure frames can be arranged such that OFDM symbols in    different physical layer frames may have difference subcarrier    spacing. Frequency domain frame synchronisation (preamble detection)    is thus not readily possible. The preamble symbol must therefore be    detected in the time domain. It is only after the preamble is    decoded and its signalling payload interpreted that frequency domain    processing of the frame can proceed because only then would the    receiver have knowledge of the OFDM parameters (number of    sub-carriers, guard interval) etc of the data payload bearing OFDM    symbols in the body of the frame.-   5. Robustness; The preamble should be detectable and decodeable by    all receiver types under all channel conditions where such receivers    are expected to work. This means that the preamble should be robust    to both high levels of noise, low signal to noise ratios and high    levels of Doppler shift as experienced during reception on the move.    Robustness to high levels of noise constrains the maximum    transmission parameters for coding and modulation (MODCOD) that can    be used for carrying the signalling payload of the preamble whilst    robustness to Doppler constrains the minimum sub-carrier spacing of    the preamble OFDM symbol. The preamble OFDM symbol must use a    sub-carrier spacing that is large enough to be reasonably resilient    to a high Doppler spread. Furthermore, the preamble OFDM symbol    should also allow decoding in the presence of frequency shift,    common phase error, maximum expected multipath delay spreads etc.

As explained above the preamble OFDM symbol conveys signalling datawhilst the OFDM symbols within the body of the transmission frame conveypayload data as shown in FIG. 5. Each transmission frame shown in FIG. 5has particular characteristics. A data bearing frame 100 carries a frameof data, which may use a higher operating mode providing a higher numberof sub-carriers per OFDM symbol, for example, approximately 32 thousandsub-carriers (32k mode) thereby providing a relatively high spectralefficiency, but requiring a relatively high signal to noise ratio toachieve an acceptable data integrity in the form of the bit error rate.The higher order operating mode would therefore be most suitable tocommunicate to stationary television receivers which have sensitivedetection capabilities including well positioned fixed antenna forrecovering audio/video data from the 32k OFDM symbols. In contrast, theframe structure also includes a second frame 102 which is generated tobe received by mobile television receivers in a more hostile radiocommunications environment. The frame 102 may therefore be arranged toform payload hearing OFDM symbols with a lower order modulation schemesuch as BPSK or QPSK and a small or lower number of sub-carriers perOFDM symbol (FFT size) such as 4K or 8K to improve the likelihood that amobile receiver may be able to receive and recover the audio/video datain a relatively hostile environment. In both the first frame 100 and thesecond frame 102 a preamble symbol 104, 106 is provided which providessignalling parameters for detecting the audio/video data transmitted inthe payload part of the transmission frame 100, 102. Similarly, apreamble symbol 108, 110 is provided for a future extension frame 112.

Design of New Preamble Symbol

Some example embodiments can provide an arrangement for forming apreamble symbol for use for example with the transmission frames shownin FIG. 5 in which there is an improved likelihood of detecting thepreamble symbol particularly in harsh radio environments. Furthermore,the framing structure shown in FIG. 5 can be devised such that thenumber of sub-carriers of the payload bearing OFDM symbols is differentfrom frame to frame and furthermore, these sub-carriers may usedifferent modulation schemes. Thus the OFDM symbols which carry thepayload data may be of a different type to the OFDM symbols carrying thesignalling data. An example block diagram of a part of the transmittershown in FIG. 2 for transmitting the signalling data is shown in FIG. 6.

In FIG. 6 the signalling data is first fed to a scrambling unit 200which scrambles the signalling data which is then fed to a forward errorcorrection (FEC) and modulator unit 202 which encodes the signallingdata with a forward error correcting code and then interleaves it beforemapping the encoded data onto π/4-BPSK modulation symbols. A pilotinsertion unit 204 then inserts pilots in between modulation symbols toform the OFDM symbols of the preamble 104, 106, 108, 110. The OFDMsymbol forming the preamble is then scaled by a scaling unit 206 inaccordance with a predetermined factor (1−G). The scaling unit 206adapts the transmission power of the preamble with respect to asignature sequence which is combined with the OFDM symbols of thepreamble before transmission so that the total transmission power of thepreamble remains the same as it would have been without the signaturesequence.

According to the present the technique a signature sequence generator208 is configured to generate a signature sequence which is fed to asecond scaling unit 210 which scales the signature sequence by apredetermined factor G before the scaled signature sequence is combinedwith the OFDM symbol of the preamble by a combining units 212. Thus thesignature sequence W(k) is combined with the OFDM symbol in thefrequency domain so that each of the coefficients of the signaturesequence is added to one of the subcarrier signals of the OFDM symbol.The combined preamble OFDM symbol and signature sequence are thentransformed from the frequency domain to the time domain by an inverseFourier transform processor (IFFT) 214 before a guard interval insertionunit inserts a time domain guard interval. At an output of the guardinsertion unit 216 the preamble symbol is formed on output channel 218.

As can be seen for the example shown in FIG. 6 the signature sequence iscombined with the OFDM symbol carrying signalling data in the frequencydomain so that a spectrum of the preamble symbol after combining remainswithin a spectral mask for the transmission channel. As will beappreciated for some examples the signature sequence may be combinedwith the OFDM symbol in the time domain. However other bandwidthlimiting processes must then be introduced after the combination of thesignature sequence with the preamble OFDM symbol in the time domainwhich may affect the correlation properties of the signature sequence atthe receiver.

In the example illustration in FIG. 6, the scrambling of the signallingdata by the scrambling unit 200 ensures that the peak-to-average powerratio (PAPR) of the preamble symbol will not be excessive due to manysimilarly modulated OFDM sub-carriers. The scrambled signalling bits arethen forward error correction encoded by the FEC and BPSK unit 202 witha 4K LDPC code at a low code rate (1/4 or 1/5) prior to mapping toπ/4-BPSK which is a low order constellation within the unit 202. Thepilots inserted at this stage by the pilot insertion unit 204 are notfor channel estimation, but for frequency offset estimation as will beexplained shortly. At this stage, a complex preamble signature sequencealso composed the same number of complex samples as the usefulsub-carriers as the OFDM symbol is added to the samples of thesignalling OFDM symbol by the combiner 212. When generated, eachpreamble signature sequence sample is a point on the unit circle butbefore addition to the preamble OFDM symbol, each sample is sealed by apredetermined factor G, by a scaler 210 and the corresponding OFDMsymbol sample is scaled by (1−G) by a scaler 206 so that the power ofthe composite preamble symbol should be the same as the power of thesignalling OFDM symbol at point A in FIG. 6.

The IFFT 214 then forms the OFDM symbol in the time domain, which isthen followed by the insertion of the guard interval by the guardinsertion unit 216 which prepends the Ng samples of the preamble OFDMsymbol at the start of the preamble OFDM symbol—also known as the as acyclic prefix of the preamble OFDM symbol. After guard intervalinsertion, a preamble OFDM time domain symbol of duration Ts=Tu+Tg madeup of Ns=Nu+Ng complex samples where Tu is the useful symbol period withNu samples and Tg is the guard interval duration with Ng samples isformed.

The Signature Sequence Generator

As explained above, the preamble symbol generator of FIG. 6 generates asignature sequence which is combined with the signalling OFDM symbol(first OFDM symbol), which forms the preamble symbol of the frame, inorder to allow a receiver to detect the preamble at lower signal tonoise ratios compared to signal to noise ratios which are required todetect and recover data from OFDM symbols carrying the payload data. Thesignature sequence generated by the signature sequence generator 208 canbe formed using two pseudo random bit sequence generators one for thein-phase and other for the quadrature phase component. In one examplethe signature sequences are constant amplitude zero autocorrelation(CAZAC) or Zadoff-Chu sequences. In other examples each signaturesequence is formed from a pair of Gold code sequences chosen because oftheir good auto-correlation properties, or other example signaturesequences could be used such as M-sequences.

One example of the signature sequence generator 208 shown in FIG. 6 isshown in more detail in FIG. 7. FIG. 7 is arranged to generate a complexsignature sequence which is added to the complex samples of thesignalling OFDM symbol by the combiner 212 shown in FIG. 6.

In FIG. 7 two linear feedback shift registers are used in each case togenerate a pair of pseudo random bit sequences for the in-phase 300.1and 300.2 and quadrature 302.1 and 302.2 components. In each case, thepseudo-random bit sequence pair is combined using exclusive-OR circuits310, 312 to produce the Gold sequences for the in-phase (300.1 and300.2) and quadrature (302.1 and 302.2 ) part of the signature sequence,respectively. A binary to bipolar mapper unit 314, 316 then formsrespectively a sample for the in-phase 318 and quadrature (imaginary)320 components of the signature sequence. Effectively, the arrangementshown in FIG. 7 generates Gold codes formed by XORing two m-sequences.The m-sequences are generated by the linear feedback shift registers300, 302. A table 1 below shows the generator polynomials for the linearfeedback shift registers according to the example shown in FIG. 7:

TABLE 1 Generator polynomials for complex signature sequence. SequenceName Generator polynomial R_seq1 x¹³ + x¹¹ + x + 1 R_seq2 x¹³ + x⁹ +x⁵ + 1 I_seq1 x¹³ + x¹⁰ + x⁵ + 1 I_seq2 x¹³ + x¹¹ + x¹⁰ + 1

Determining an Optimum Value for the Scaling Factor G

As shown in FIG. 6, the scaler 210 multiplies the signature sequence bya factor G and the scaler 206 multiplies the signalling OFDM symbol by afactor 1−G. As such, if the time domain signalling OFDM symbol signal isc(n) while the signature sequence signal is f(n), then the compositetransmitted preamble symbol s(n) is given by:

s(n)=(1−G)c(n)+Gf(n)

where G is the scaling applied to the signature sequence. The signaturesignal effectively adds distortion to the signalling OFDM symbol therebyincreasing the bit error rate of the signalling OFDM symbol at thereceiver. Furthermore, with a normalised power of 1, the compositesymbol in effect distributes power between the signature signal and thesignalling OFDM symbol signal. With a high value for G, the signaturesignal has more power and so frame synchronisation (detection of thepreamble) at the receiver should be achieved at a lower signal to noiseratio. However, reducing the power of the signalling OFDM symbol (inorder to increase the power of the signature signal) also means thaterror-free decoding of the signalling information itself becomes moredifficult at the receiver as the signal-to-noise of the signalling OFDMsymbol has fallen. Therefore, an optimum value for G has to be acompromise between these conflicting aims. We can further defineP=(1−G)/G which is proportional to the power ratio between thesignalling OFDM symbol and the signature signal. An appropriate valuefor G can be set by experimenting with this power ratio P.

The performance of example error correction codes which may be used forprotecting the preamble symbol can be assessed in the presence ofAdditive White Gaussian Noise, using an appropriate constellation forthe signalling information. For example a QPSK modulation scheme can beused with example error correction codes. In the present example 4K LDPChalf rate and quarter rate codes were evaluated. FIG. 8 provides agraphical illustration of the performance for communicating thesignalling data using the signalling OFDM symbol for these half andquarter rate LDPC codes and shows for each code a hit error rateperformance with respect to signal to noise ratios for an additive whiteGaussian noise channel. It can be seen that at a signal to noise ratioof −3 dB and a signal to noise ratio of 1 dB, the quarter rate and halfrate codes respectively each become error free. These values of signalto noise ratios were then increased to −2 dB and 2 dB respectively andthen the signature signal added with values of P varied until a biterror rate of zero was achieved.

As will be appreciated the error correction code which may be used toprotect the signalling data carried in the preamble symbol may havecoding rates which are different to rate one-half and rate one-quarter.In some embodiments the coding rate is less than or equal toone-quarter. In one example the coding rate is one-fifth (⅕).

FIG. 9 provides a graphical plot for code rates of one quarter and onehalf showing a bit error rate for each code rate as the factor P on thex-axis and SNR fixed to −2 dB and 2 dB respectively. As can be seen fromthese results setting P=8 dB will give a bit error rate close to zero,despite the presence of the signature sequence, which has been added tothe signalling OFDM symbol. It can also be seen experimentally, thatwith this value of the factor P, preamble detection can be achieved. Avalue of P=8 dB has, therefore, been adopted for the different half andquarter rate code rates with QPSK modulated data subcarriers of thesignalling OFDM symbol. As can be seen an optimising choice for thefactor P can be chosen from the results produced.

Determining a Suitable Guard Interval Fraction

According to example embodiments of the present technique, the samepreamble symbol will delimit physical layer frames meant for both fixedand mobile reception. In the following analysis it is assumed that abroadcast transmission system, which has both types of transmissionframes will be used. As such one of the principal factors affecting thereception of payload data bearing OFDM symbols transmitted for fixedreception is spectral efficiency. As explained above, this means the useof large numbers of sub-carriers for the OFDM symbols andcorrespondingly large FFT sizes because a smaller guard intervalfraction (GIF) can be used to get a large guard interval duration (GID).A large GID can allow a broadcast system to have a greater separationbetween broadcast transmitters and can operate in environments with agreater delay spread. In other words the broadcast transmission systemis configured with a wider spacing between transmitters forming a singlefrequency network (SFN).

FIG. 10 illustrates how the selection of the guard intervals can beaffected when different operating modes providing different numbers ofsub-carriers per OFDM symbol (different FFT sizes) are used fordifferent frames in the same transmission. In contrast to the diagramshown in FIG. 5, the diagram shown in FIG. 10 is in the time domain.Three sets of OFDM symbols are shown in the time domain illustrative ofwhat may happen at the point where one frame ends and another starts ina single transmission. In FIG. 10a the duration of the last OFDM symbol402 of the ending frame is the same as that of the first OFDM symbol 404of the starting frame. The unshaded area 405 between the two OFDMsymbols 402 and 404 represents the guard interval that precedes symbol404. In FIG. 10b an example of a preamble symbol shown as the light greyarea 406 is inserted to delimit the two frames. As can be seen, thisexample preamble symbol 406 has a shorter duration than the data bearingsymbols 402 and 404 as a consequence of having a different number ofsub-carriers per OFDM symbol. Accordingly, if the GIF for the preamblesymbol is the same as for the data symbols, the guard interval durationfor the preamble symbol will not be as long as that of the data bearingsymbols. Accordingly, if the delay spread of the channel is as long asthe guard interval of the data bearing OFDM symbol 402, then thepreamble symbol 406 will suffer inter-symbol interference from the lastsymbol 402 of the previous frame. Examples shown in FIG. 10c can providean arrangement in which the guard interval fraction for the preamblesymbol is selected to the effect that the guard interval duration of thepreamble symbol 406 matches or may be longer than the guard intervalduration of the last data bearing symbol 402 of the previous frame.

According to some example embodiments the largest number of sub-carriersper symbol is substantially thirty two thousand (32K). With a 32K FFTsize in DVB-T2 for example, the largest GIF is 19/128. For 6 MHz channelraster, this represents a GID of about 709.33 us. When this GID is usedfor the frame carrying OFDM symbols targeted for fixed receivers, thepreamble OFDM symbol GID should at least be of a similar value,otherwise, the preamble symbol will suffer inter-symbol-interferencefrom the last symbol of a previous fixed reception frame.

In a 6 MHz channel raster system in which for example DVB-T2 istransmitted, an OFDM symbol having substantially four thousandsub-carriers (4K) OFDM symbol has a duration of only 2*224*8/6=597.33us. Therefore even with a GIF=1, it is not possible to get a GID of709.33 us with a 4K OFDM symbol. A table below lists possible operatingmodes that are receivable in medium to high Doppler frequencies (for themobile environment) and some possible guard intervals.

TABLE 2 Mobile FFT modes and their possible guard intervals FPT Size Tuin 6 MHz (us) GIF GID (us) Ts (us) 4K 597.33 1 597.33 1194.667 ¼ 298.671493.338 8K 1194.67 ½ 597.33 1792.005 19/32 709.33 1904.000 ¾ 896.002090.638

From the above table it can be seen that only an 8K operating mode forthe preamble OFDM symbol has GIF <1 which matches or exceeds the maximumGID for a 32K maximum number of sub-carriers of the OFDM symbol. Inconclusion therefore, embodiments of the present technique can provide anumber of sub-carriers for the signalling or preamble OFDM symbol of8192 sub-carriers, which corresponds to an 8K FFT size, for which theGIF will be about 19/32. This means that the total signalling OFDMsymbol will have a duration of Ts≈1904 us. Furthermore an 8K operatingmode will have a sub-carrier spacing, which provides a mobile receiverwith a reasonable chance of detecting and recovering the signalling datafrom the preamble OFDM symbol in medium to high Doppler frequencies. Itcan be understood that in embodiments of this disclosure, the GIF of thepreamble symbol has to be chosen to have a GID that is the same orlonger than the longest GID of the maximum FFT size available in thesystem.

Channel Estimation Considerations

As known in OFDM transmission systems such as DVB-C2, frequency domainpreamble pilots may be inserted into a preamble symbol at regularintervals for use in channel estimation prior to equalisation of thepreamble symbol. A density of such pilots Dx, which is the spacing infrequency is dependent on the maximum delay spread that can be expectedon the channel. As explained above, with a single frequency transmissionnetwork, it can be advantageous to use a larger GID. For such singlefrequency networks, a channel impulse response can have a duration whichis equal to the GID. Thus, the delay spread of the channel for preambleequalisation may be as much as the GID. When using preamble pilotsspaced by Dx subcarriers, pilot-aided channel estimation is possible fordelay spreads as long as Tu/Dx. This means that Dx must be set suchthat:

T _(u) /D _(x) ≧T _(g)

Since for an 8K preamble in a 6 MHz channel, Tu=1194.67 us,

$D_{x} \leq \lceil \frac{T_{s}}{T_{g}} \rceil$

Substituting Tu=1194.67 and Tg=709.33, D_(x)≦2. This means that morethan one in every two sub-carriers of the signalling OFDM symbol wouldbecome a pilot sub-carrier. This would have the effect of cutting thecapacity of the signalling OFDM symbol by more than half. As such, thisconclusion suggests that an alternative technique should be adopted toestimate the channel impulse response rather than using frequency domainpilots.

Frequency Offset Considerations

At first detection, the signalling or preamble OFDM symbol may have tobe decoded in the presence of any tuning frequency offsets introduced bytuner 52. This means that either the signalling data should be modulatedunto the preamble OFDM symbol in a manner that reduces the effects ofany frequency offsets or resources are inserted into the preamble symbolto allow the frequency offset to be estimated and then removed prior topreamble decoding. In one example the transmission frame may onlyinclude one preamble OFDM symbol per frame so the first option isdifficult to achieve. For the second option, additional resources can bein the form of frequency domain pilot subcarriers, which are insertedinto the OFDM so that these can be used to estimate the frequency offsetand common phase error. The frequency offsets are then removed beforethe symbol is equalised and decoded. In a similar vein to the insertionof pilots into the data payload bearing OFDM symbols, embodiments of thepresent technique can be arranged to provide within the signalling(preamble) OFDM symbol pilot sub-carriers, which can allow for theestimation of frequency offsets that are larger than the preamblesubcarrier spacing. These pilots are not spaced regularly in thefrequency dimension to avoid instances when multipath propagation mayresult in regular nulls of the pilots across the full preamble OFDMsymbol. Accordingly, 180 pilot sub-carriers can be provided across the8K symbol with the positions defined aprori. The sub-FFT bin frequencyoffset is estimated via the detection of the preamble OFDM symbolitself. Accordingly embodiments of the present technique can provide apreamble OFDM symbol in which the number of sub-carriers carrying pilotsymbols is less than the number which would be required to estimate achannel impulse response through which the preamble OFDM symbol istransmitted, but sufficient to estimate a coarse frequency offset of thetransmitted OFDM symbol.

Frequency Offset Detection at the Receiver

As explained above the preamble is formed by combining an OFDM symbolcarrying signalling data with a signature sequence. In order to decodethe signalling data, the receiver has to first detect and capturepreamble OFDM symbol. In one example the signature sequence may bedetected using a matched filter which has impulse response which ismatched to the conjugate of the complex samples of the known signaturesequence. However any frequency offset in the received signal has aneffect of modulating the output of the matched filter and preventingaccurate detection of the signature sequence using a match filter. Anexample receiver for detecting the preamble and recovering thesignalling information provided by the preamble in the presence of afrequency offset is shown in FIG. 11 a. In FIG. 11 a, a signal receivedfrom an antenna is converted to a baseband signal, using a conventionalarrangement as shown in FIG. 4 and fed from an input 420 respectively toa complex number multiplier 422 and a frequency synchroniser 424. Thefrequency synchroniser 424 serves to detect the frequency offset in thereceived signal r(x) and feed a measure of the offset in respect of anumber of subcarriers to an oscillator 426. The oscillator 426 generatesa complex frequency signal which is fed to a second input of themultiplier 422 which serves to introduce a reverse of the offset intothe received signal r(x). Thus the multiplier 422 multiplies thereceived signal r(x) with the output from the oscillator 436 therebycompensating or substantially reversing the frequency offset in thereceived signal so that a preamble detection and decoding unit 430 candetect the preamble OFDM symbol and recover the signalling data conveyedby the preamble which is output on output channel 432.

FIG. 11b provides an example implementation of the frequencysynchroniser 424 which forms part of the receiver shown in FIG. 11a . InFIG. 11b the received signal is fed from the input 420 to a preambleguard interval correlator 432 which generates at a first output 434 asignal providing an indication of the start of the useful part of theOFDM symbol samples Nu. A second output 436 feeds the samples of theOFDM symbol to a Fourier transform processor 438, but delayed by thenumber of samples in the useful part Nu. The first output 434 from thepreamble guard interval correlator 432 detects the location of the guardinterval and serves to provide a trigger signal from a thresholddetector 440 to the FFT 438 through a channel 442 which triggers the FFT438 to convert the time domain samples of the useful part of the OFDMsymbol Nu into the frequency domain. The output of the Fourier transformprocessor 438 is fed to a continuous pilot (CP) matched filter unit 444,which correlates the pilot signals in the received OFDM symbol withrespect to replicas at the receiver which are used to set an impulseresponse of the CP matched filter in the frequency domain. The matchedfilter 444 therefore correlates the regenerated pilots with the receivedOFDM symbol and feeds a result of the correlation to an input to adetection threshold unit 446. The detection threshold unit 446 detectsan offset in the received signal in terms of the number of FFT bins onchannel 448 which effectively provides the frequency offset which is fedto the oscillator 426 for correcting the offset in the received signal.

FIG. 11c provides an example of implementation of the preamble guardinterval correlator 432 and corresponds to a conventional arrangementfor detecting the guard interval. Detection is performed by crosscorrelating the samples of the received OFDM symbol with themselvesafter a delay of Nu samples with the cross correlation outputsaccumulated over consecutive Ng sample intervals. Thus the receivedsignal is fed from an input 420 to a multiplier 450 and a delay unit 452which feeds an output to a complex conjugator 454 for multiplying by themultiplier 450 with the received signal. A delay unit 456 delays thesamples by the number of samples Ng in the guard interval and a singledelay unit 458 delays an output of an adder 460. The adder 460 receivesfrom the multiplier 450 the results of multiplying the received signalwith a conjugate of the delayed samples corresponding to the usefulsamples Nu which is then fed to the adder 460. Together adder 460, delayblocks 456 and 458 implement a moving average filter of order Ng whoseeffect is to accumulate successive output of the cross-correlator overNg samples. Thus at a point 434 there is provided an indication of thedetection of the useful part of the OFDM symbol by detecting the guardinterval period. The output 436 provides the delayed received signalsamples which are fed to the FFT in order to trigger the Fouriertransform after the guard interval has been detected by the first output434.

FIG. 11d provides another example of implementation of the frequencysynchroniser 424 and corresponds to a first detection of the preamblesymbol by use of a signature sequence matched filter 462. Firstlyhowever, the differential encoder block 461 is used to alter thereceived signal so as to reduce the modulation of the matched filteroutput by any frequency offset present in the received signal. Thedifferential encoder 461 is applied both to the received signal and thetime domain signature sequence which is generated by inverse Fouriertransform 506 of the output of the frequency domain signature sequencegenerator 504. The signature sequence matched filter 462 to be describedlater in FIG. 15a is a finite impulse response filter whose taps are setto the coefficients of the time domain signature sequence.

The circuit shown in FIG. 11d therefore forms an example of thefrequency synchroniser 424 in which the signature sequence generator 504re-generates the signature sequence, the inverse Fourier transformer 506transforms the signature sequence into the time domain, and thedifferential encoder 461 compares differentially successive samples ofthe received signal to reduce a modulating effect of the frequencyoffset in the radio signal, and correspondingly compares differentiallysuccessive samples of the time domain version of the signature sequence.As already explained, the matched filter 462 has an impulse responsecorresponding to the differentially encoded signature sequence andreceives the received signal from the differential encoder 461 andfilters the differentially encoded received signal to generate at anoutput an estimate of the coarse frequency offset.

Corresponding to output channel 434 in FIG. 11 b, output channel 463 inFIG. 11d produces a signal which is fed to the threshold block 440 togenerate a trigger for the FFT 438; whilst output channel 436 in FIG.11b corresponds to output channel 464 in FIG. 11d . This channel conveysthe preamble OFDM symbol samples to the FFT block 438 which at the rightmoment is triggered by through channel 442 by the threshold block 440.FIG. 11e provides an example of the differential encoding block 461. Thereceived samples r(n) enter a unit delay element 465 and also aconjugation block 466. The delay element 465 delays each sample for onesample period while the conjugation element 466 changes each inputsample to its conjugate at its output whose effect is to convert aninput [r_(i)(n)+jr_(q)(n)] into an output [r_(i)(n)−jr_(q)(n)]. Thisconjugated sample is then subtracted from the output of delay element465 by the adder 467. For an input signal [r_(i)(n)+jr_(q)(n)] andoutput [y_(i)(n)+jy_(q)(n)]n=0, 1, 2 . . . , the differential encoder461 acts to implement the equation:

[y _(i)(n)+jy _(q)(n)]=[r _(i)(n−1)−r _(i)(n)]+j[r _(q)(n−1)+r _(q)(n)]

Accordingly before preamble detection and decoding is performed by thepreamble detection decoding unit 430 the frequency offset in thereceived signal is estimated and corrected by the arrangements shown inFIGS. 11a and 11b and 11 c, or 11 d and 11 e.

Preamble Detection and Decoding at the Receiver

As explained above for the example of the receiver shown in FIG. 11 a, apreamble detector and decoder 430 is configured to detect the preamblesymbol and to recover the signalling data from the preamble symbol. Tothis end, the preamble detector and decoder 430 detects the preamble bydetecting the signature sequence and then removes the signature sequencebefore recovering the signalling data from the preamble. Exampleembodiments of the preamble detector and decoder 430 are illustrated inFIGS. 12, 13 and 14.

Embodiments of the present technique can provide a receiver whichdetects the signature sequence and removes the signature sequence in thefrequency domain or in the time domain. FIG. 12 provides a first examplein which the signature sequence is removed in the frequency domain.Referring to the example receiver shown in FIG. 11 a, the received baseband signal is fed from a receive channel 428 to a matched filter 502and a demodulator 550. The match filter 502 receives the signaturesequence in the time domain after a signature sequence generator 504,which is the same as the signature sequence generator 212 at thetransmitter, re-generates a copy of the signature sequence. The matchedfilter 502 is configured to have an impulse response which is matched tothe time domain signature sequence. As such, it correlates the timedomain signature sequence with the received signal fed from the receivechannel 428 and the correlation output result can be used to detect thepresence of the preamble OFDM symbol when an output of the correlationprocess exceeds a predetermined threshold. Furthermore, as a result ofthe presence of the signature sequence in the preamble OFDM symbol, animpulse response of the channel through which the received signal haspassed can also be estimated from the correlation output of the matchedfilter by a channel impulse response estimator 508. The receiver cantherefore include an arrangement for estimating the channel impulseresponse using the signature sequence without recourse to thetraditional scattered pilots.

Having detected the presence of the signature sequence and estimated thechannel impulse response, the effect of the channel impulse response canbe removed from the received signal within the demodulator 550.Accordingly a Fast Fourier Transformer 518 transforms the channelimpulse response estimate into the frequency domain channel transferfunction and feeds the channel transfer function to an equaliser 516within the demodulator 550.

In the receiver shown in FIG. 12 the demodulator 550 is arranged torecover the signalling data in a base band form encoded with an errorcorrection code. The demodulator 550 therefore recovers the signallingdata from the signalling (preamble) OFDM symbol, which is then decodedusing a forward error correction decoder 520 before being descrambled bya descrambling unit 522 which corresponds to the scrambling unit 200shown in FIG. 6 but performs a reverse of the scrambling.

The demodulator 550 includes a guard interval remover 512, which removesthe guard interval from the signalling OFDM symbols, and an FFT unit514, which converts the time domain samples into the frequency domain.The equaliser 516 removes the effects of the channel impulse response,which has been converted into the frequency domain to form a channeltransfer function by the FFT unit 518 as already explained above. In thefrequency domain the equaliser 516 divides each signalling data carryingOFDM sub-carrier by its corresponding channel transfer coefficient toremove, as far as possible, the effect of the transmission channel fromthe modulation symbols.

A signature sequence remover is formed by an adder unit 519 whichreceives the signature sequence in the frequency domain generated by thesignature sequence generator 504 after this has been scaled by thescaling factor G, as explained above by a scaling unit 521. Thus thesignature sequence remover 519 receives at a first input the equalisedpreamble OFDM symbol and on a second input a scaled signature sequencein the frequency domain and subtracts one from the other to form at theoutput estimates of the modulation symbols which were carried by thedata bearing subcarriers of the preamble OFDM symbol.

The modulation symbols representing the error correction encodedpreamble signalling data are then demodulated and error correctiondecoded by the demodulator and FEC decoder 520 to form at an output thescrambled bits of the L1 signalling data which are then descrambled bythe descrambling unit 522 to form as an output 524 the L1 signallingdata bits.

A further example of the preamble detector and decoder 430 whichoperates in the time domain to remove the signature sequence is showingin FIGS. 13 and 14. FIG. 13 provides an example of the preamble detectorand decoder 430 which corresponds to the example shown in FIG. 12 and soonly differences with respect to the operation of the example shown inFIG. 13 will be explained. In FIG. 13 as with the example in FIG. 12 thebaseband received signal is fed to a signature sequence matched filter502 and to a demodulator 550. As with the example shown in FIG. 12, thesignature sequence matched filter cross-correlates the received signalwith an impulse response which is matched to the time domain signaturesequence. The signature sequence is received in the time domain form byregenerating the signature sequence in the frequency domain using thesignature sequence generator 504 and transforming the signature sequenceinto the time domain using an inverse Fourier transform processor 506.As with the example shown in FIG. 12 a channel impulse responseestimator 508 detects the channel impulse response from the output ofthe signature sequence matched filter 502 and forms this into thefrequency domain channel transfer function using an FFT unit 518 to feedthe frequency domain channel estimate to an equaliser 516 within thedemodulator 550.

So far the operation of the example shown in FIG. 13 corresponds to thatshown in FIG. 12. As shown in FIG. 13 the demodulator 550 includes thesignature sequence remover 559 at before the guard remover 512. The timedomain signature sequence which is fed from the inverse Fouriertransform unit 560 is sealed by the scaling unit 521 by thepredetermined factor G. The sealed time domain signature sequence isthen fed to the signature sequence remover 559 which removes thesignature sequence in the time domain from the received baseband signal.Thereafter the guard remover 512, the FFT unit 514 and the equaliser 516operate in a corresponding way to the elements shown in FIG. 12.

The signature sequence remover 559 shown in FIG. 13 is shown in moredetail in FIG. 14. In FIG. 14 the signature sequence remover 559comprises a guard interval inserter 561, a combiner unit 560 and an FIRfilter 562. The time domain baseband received signal is received on theinput channel 428 at one input of the combiner unit 560. A second input564 receives the scaled time domain version of the signature sequence,which is fed to the guard interval inserter 561 which prepends a cyclicprefix to the signature sequence in much the same way as the guardinterval inserter 561 42 at the transmitter. The output of the guardinterval inserter feeds the FIR filter 562 which receives on a secondinput 566 the estimate of the channel impulse response generated thechannel impulse response extraction block 508. 502. The FIR filter 562therefore convolves the channel impulse response estimate with thesignature sequence in the time domain which is then subtracted by thecombiner 560 from the received baseband signal to remove the effect ofthe signature sequence from the received signal. FIG. 15b shows a moredetailed example implementation of this signature sequence removal andhow the FIR filter 562 is configured.

As will be appreciated the operation of the demodulator and FEC decoder520 and the scrambler 522 perform the same functions as explain withreference to FIG. 12.

Matched Filter

As indicated above the matched filter 502 generates an output signalwhich represents a correlation of the received signal with the signaturesequence. A block diagram showing an example of the signature sequencematched filter 502 is shown in FIG. 15 a.

FIG. 15a shows a sequence of Ns delay elements 600 connected to scalingunits 602 which scale each of the samples of the data stored in thedelay storing unit 600 by a corresponding component of the signaturesequence P(n) but conjugated. The output from each of the sealing units602 is then fed to an adding unit 604 which forms an output signalrepresenting a correlation of the received signal samples r(n) with thesignature sequence at an output 606. The matched filter implements theequation:

g(i)=Σ_(n=0) ^(N) ^(s) ⁻¹ P*(n)r(n+i) for i=−Ns+1, −Ns+2 . . . , 0,1,2,. . . Ns−1

When the filter taps P(i) are of form (±1±j1), the multiplier at eachtap could simply be done by adding and subtract circuits for each of thein-phase and quadrature components. When the signature sequence is aCAZAC sequence, the quadrature components of P(i) are not bipolar. Thescaling units 602 can use the sign of each quadrature component insteadso as to have the form (±1±j1).

FIG. 16a and FIG. 16b provide examples of a correlation output of thematch filter for a multipath environment. In this case the channel iscomposed of three paths and the preamble is a 4K symbol with GIF of ¼for illustrative purposes only. As can be seen there is a clearcorrelation peak when the signature sequence of the received signalcoincides with the signature sequence at the receiver. The example shownin FIG. 16b shows the output of the matched filter but with a moreexpanded x-axis showing an increase in resolution which is expanded fromthe correlation peak shown in FIG. 16 a. For this channel, there arethree paths as tabulated in the Table below:

TABLE 3 Multipath profile of example channel Path Delay(us) [samples]Power(dB) 1 0 [0] 0 2 10 [68] −10 3  25 [171] −6

Channel Impulse Response Extractor

As can be seen from FIG. 16 b, both the amplitudes of the main impulsesand their relative delays coincide with the characteristics of themultipath channel profile through which this particular signalpropagated. To detect the actual channel paths, a threshold of energydetection is set to an appropriate multiple of the root mean square(RMS) level of the matched filter output within a window ±Ns of thehighest amplitude output sample. The exact multiple of the RMS is chosenexperimentally depending on the lowest signal to noise ratio under whichthe system is to work. Any sample of the matched filter output abovethis threshold is taken as a channel path, and all other samples arethen set to zero in the channel impulse estimator 508. Finally, thechannel impulse response (CIR) is normalised by dividing all its sampleswith the highest amplitude sample. In this way, the relative amplitudesand delays of each of the impulses in the channel through which thereceived signal has passed can be estimated.

Signature Sequence Remover

Having formed an estimate of the channel impulse response, a componentof the received signal corresponding to that contributed by thesignature sequence in the received signal can be generated by passingthe received signal r(i) through the signature sequence remover 559,which is configured with filter taps h_(n) to reflect the delay andamplitude profile of the channel impulse response. This can beaccomplished by suitable scaling, shifting and adding of the signaturesequence of length Ns=Nu+Ng of the preamble symbol. An example of thefilter is shown in FIG. 15 b.

As shown in FIG. 15 b, the signature sequence remover 559 includes afinite impulse response (FIR) filter 562 made up of a delay linecomprised of Ns−1 delay elements 652.1, 652.2, to 652.Ns−1. The outputof these delay elements are connected to corresponding gain terms 651.1,651.2, to 651.Ns−1 each of which gain stages feed their output to theadder 653. The input 654 of the filter is connected both to the input ofdelay element 652.1 and to the input of gain term 651.0. The output 656of the FIR filter 650 is connected to the input of an adder 560 whoseother input 657 receives the received preamble signal samples r(i).During operation, the gain stages of the FIR filter are set to thenegative values of the samples of the channel impulse response derivedby the channel impulse response estimator 506. The FIR 650 generates atan output 656 a signal representing the convolution of the signaturesequence by the channel impulse response estimate, which effectivelyprovides an estimate of the effect of the channel on the signaturesequence imposed upon the signalling OFDM symbol. An adder 560 thensubtracts the output signal of the FIR 656 from the received signal froman input 657 to remove the effect of the signature sequence from thereceived signal to form an output 660. Therefore a result (of thesignature sequence transiting the channel described by the channelimpulse response) is subtracted from the received signal by thesignature sequence remover 510 with a delay matched to the point fromwhich the first significant impulse (of the output of the matchedfilter) occurred. This process can be iterated in that the matchedfilter 502 can be re-run with the results of the subtraction, thechannel impulse response re-estimated by the channel impulse responseestimator 508 and the its effect on the signature sequence beingextracted again by the signature sequence remover 559. As a result, amore accurate estimate of the effect of the signature sequence on thereceived signal can be estimated and subtracted from the receivedsignal. Channel impulse responses from all iterations can then be summedand normalised to provide an improved estimate of the channel impulseresponse from which the channel transfer function (CTF) is derived forpreamble symbol equalisation.

Frequency Offset Estimation

FIG. 17 provides a more detailed schematic block diagram of the preamblepilot matched filter 444 used for detecting a coarse Frequency offset inthe received signalling OFDM symbol, which may form part of thefrequency synchroniser 424 of FIG. 11 a. As explained above, the numberof pilots introduced into the signalling OFDM symbol is less than thenumber which would be required in order to estimate the channel. Thenumber of pilot symbols is therefore set to estimate a coarse frequencyoffset. The block diagram shown in FIG. 17 provides an examplerepresentation of the coarse frequency remover 513 and is shown withthree versions of the received preamble signal 701.

As shown in FIG. 17 a sequence of delay elements 700 are used to feed indiscrete samples of the signal which are then multiplied by multipliers702 with the known pilot signal values P(n) and summed by a summing unit704 to form a correlation output 706. A pulse detector or peak detector708 is the same one shown as 446 in FIG. 11b which then generates anoutput signal on channel 710 showing a peak when there is a coincidencebetween a relative offset of the received signal with the company of thepilot signals at the receiver. Shaded circles of each received signal701 show sub-carrier cells that represent preamble pilots whilst theun-shaded cells show non-pilot sub-carrier cells. All sub-carrier cellsare shifted into the transversal filter from right to left. Theparameter MaxOff is a design parameter that represents the maximum valueof the frequency offset in units of sub-carrier spacing Ω that thedesigner may expect. The output of the pulse detector is only validbetween shifts (0.5(Na+Nu)−MaxOff) and (0.5(Na+Nu)+MaxOff) where Na isthe number of sub-carriers (out of a total of Nu) used in the preambleOFDM symbol. If the shifts are numbered from −MaxOff to +MaxOff then thepulse detector output will go high for the shift that corresponds to theobserved frequency offset.

Once Ω is detected, this coarse frequency is removed by shifting thesubcarriers by −Ω i.e. in the opposite direction to the frequencyoffset. This can also be removed prior to FFT in common with the finefrequency offset estimated from the preamble detection matched filter orguard interval correlation 432 by modulation with a suitably phasedsinusoid generated by the oscillator 426 in FIG. 11 a. The two frequencyoffsets can be used to start off the carrier correction loop for therest of the OFDM symbols in the frame.

FIG. 18 shows a pilot correlation result of a frequency offset in anexample plot of the input of the pulse detector for a frequency offsetof Ω=−88 in a case where MaxOff is set to 350. The pulse detector mightuse a threshold to clip this input as a detector of the presence orabsence of a substantial pulse.

Preamble Symbol Equalisation

After signature sequence removal from the received samples and thecoarse frequency offset has been adjusted, OFDM equalisation can beginwith the FFT of the received sequence. The FFT window starts from atrigger position in the FFT unit 514 corresponding to the relative delayof the first impulse in the channel impulse response estimate. If thechannel impulse response estimate duration is longer than the preambleGID, then the trigger position is altered to ensure that it starts atthe beginning of a Ng (Ng is the number of time domain samples in theguard interval of the preamble symbol) long window under which themaximum of the energy of the channel impulse response estimate falls.The Nu point FFT produces the preamble OFDM symbol in the frequencydomain with the effect of the channel superposed. Before equalisationand decoding, any frequency offsets have to be calculated and removed bythe frequency offset remover as explained above with reference to FIGS.11 a, 11 b, 11 c. This estimation uses correlation with the knownpreamble pilots to determine how far to the right or left the fullsymbol is shifted in frequency. Equalisation of the preamble OFDM symbolrequires a channel transfer function (CTF). This is derived by executinga Nu point FFT on the channel impulse response estimate by the FFT unit518. This provides a channel transfer function for all subcarriers inthe preamble OFDM symbol allowing subcarrier by subcarrier one-tapequalisation to take place. Finally, the equalised data subcarriers areextracted (pilot subcarriers discarded) and de-mapped, forward errorcorrection (FEC) decoded to provide the signalling.

Selected Results

FIG. 19 provides a graphical plot of bit error rate with respect tosignal to noise ratio for different code rates with and without theaddition of the signature sequence to the signalling OFDM symbol. Thus,two code rates are shown, rate one half and rate one quarter, each coderate including the example of the presence of the signature sequence andwithout the signature sequence. As can be seen, the results for rate onequarter show that the signalling OFDM symbol can be detected even atsignal to noise ratios of less than −2 dBs.

Two further sets of results shown in FIGS. 20a and 20b provide agraphical plot of bit error rate against signal to noise ratio in whichfor the results shown in FIG. 20a there is a 0 dB echo channel with anideal channel estimation and in FIG. 20b a multipath environment withtwo paths as illustrated in FIG. 20 c. Thus for FIG. 20b in contrast tothe result shown in FIG. 20a there is a relative degradation inperformance resulting from real channel estimation. However, as can beseen, the results are comparable.

Messaging by Choice of Signature Sequence

Embodiments of the present technique can also provide an arrangement inwhich the choice of signature sequence is in itself a signalling messagerepresenting information such as the presence or absence of a warningmessage within the layer one signalling data or payload. An example of atransmitter for generating a preamble symbol which includes signallingmessages according to the present technique is shown in FIG. 21 a.

FIG. 21a shows the transmitter presented in FIG. 6 with a furtheradaptation to adapt the transmitted preamble to convey additionalsignalling messages. Since the transmitter of FIG. 21a is based on thetransmitter described above and shown in FIG. 6 only the differenceswill be explained and the same parts as the transmitter of FIG. 6 havethe same numerical references.

As shown in FIG. 21 a, the signature sequence generator 208 forms partof a signalling sequence processor 800 which includes, with thesignature sequence generator 208 a sequence number controller 804. Theinput 802 to the signature sequence generator 208 receives the outputfrom the sequence number controller 804. The sequence number controllerinput 806 represents the message that the transmitter would like toconvey to receivers within the network. The signature sequence generator208 is configured to be able to generate one of N+1 possible sequences.A given number 0≦i≦N on the input 802 of the signature sequencegenerator 208 causes the signature sequence generator 208 to output thesequence whose cardinal number is i from amongst its set of signaturesequences. The output of one or other of the signature sequences fromgenerator 208 conveys a pre-determined message to all receivers in thenetwork that receive the signal. In one example the message representsan early warning signal (EWS). In this example, N=1. For example, whenthere is need to convey an early warning signal (EWS) to all receivers,the input 806 to the signature sequence processor 800 carries a 1.Accordingly, the sequence number controller 804 outputs ‘1’ onto input802 of the signature sequence generator 208 which effect is to cause thesignature sequence generator 208 to generate signature sequence number 1and output this to the gain block 210. When there is no EWS to beconveyed, the input 806 to the signature sequence processor 800 carriesa ‘0’. Accordingly, the sequence number controller 804 outputs ‘0’ ontoinput 802 of the signature sequence generator 208 which effect is tocause the signature sequence generator 208 to generate signaturesequence number zero and output this to the gain block 210. In thisexample, all receivers within the network detecting signature sequence‘1’ determine that this represents an EWS further information aboutwhich is carried as part of the layer one signalling data and the restin the payload of the frame. The receiver can then take further actionto decode and interpret the emergency information. On the other hand,receivers detecting signature sequence number zero would determine thatthere are no current emergencies imminent and so continue to decode anddisplay the audio-visual information in the payload of the frame.

In another example the signature sequence generated by the signaturesequence generator 208 is one of a predetermined set of sequences whichrepresent as many messages as there are signature sequences generated bythe signature sequence generator 208. In order to communicate each ofthese messages the message number of input 806 is arranged to be therequired signature sequence number which the signature sequencegenerator 208 uses to select one of the signature sequences from itspredetermined set of signature sequences. The selection of the signaturesequence is therefore representative of a different one of acorresponding predetermined set of messages which thereby conveysinformation which may be a particular warning message, such as a tsunamiwarning or may be a message for a different purpose. Each message canprovide different information. For example in a N=4 message system,message 1 could be an early warning of a possible emergency situation,such as an approaching hurricane or tsunami while message 2 could be anindication of an all-clear prior to the normal state represented bymessage 0 which requires no particular action. The early warning signalcould trigger the receiver to display a message or audible warninginstructing users of the device to evacuate a building for example. Thusa receiver could detect the message 1 and generate audible or visualoutput to the users to provide a warning. Similarly messages message 3and message 4 could provide similar broadcast information, such aspublic safety announcement, radio traffic announcements or flooding. Aswill be understood, the choice of sequence thereby represents one of themessages selected and therefore conveys information.

Returning to FIG. 21a and assuming a system with N=1 which represents asystem with only one message for example one with only ‘normaloperation’ and ‘impending disaster’ messages, the table shown in FIG.21b shows example parameters for generating the two signature sequencesrequired. To generate each sequence, the sequence generator 208 will usethe corresponding set of parameters {u, Na} in the CAZAC sequencegenerator equation shown.

FIG. 22 provides a conceptual representation of the operation of theguard insertion unit 216 when operating in combination with thesignalling sequence processor 800. As shown in FIG. 22, the OFDM symbolfor example for 8K mode which is fed to an input of the scaling unit 206is formed from samples including the useful part of the OFDM symbol 850and the guard interval samples 852. The first signature sequence 854 orthe second signature sequence 856 is selected under the control of thesequence number controller 804. The mapping of the guard interval fromthe useful part of the OFDM symbol is shown from the examples for themessage sequence and the signature sequence 854, 856.

A receiver which has been adapted in accordance with the presenttechnique to detect and decode a message provided by the messagesequence transmitted by the transmitter shown in FIGS. 21 and 22 isprovided in FIG. 23. FIG. 23 corresponds to the receiver shown in FIG.12 for the example of the frequency domain signature sequence removal.However, as will be appreciated a corresponding adaptation can be madeto the receiver which removes the signature sequence in the time domainas shown in FIGS. 13 and 14.

As shown in FIG. 23 the receiver includes a message detector 858. Themessage detector 858 is shown in more detail in FIG. 24. As shown inFIG. 24 the received signal is fed to the message detector 858 after thefrequency offset has been removed by the receiver as shown in FIG. 11 a.Thus the message detector 858 comprises first and second branches 860,862 in which two matched filters are present 864, 866. The first matchedfilter 864 corresponds to the matched filter 502 shown in FIGS. 12 and13 and has an impulse response which is matched to that of the signaturesequence ‘0’ for detecting the preamble symbol in ‘normal operation’.The second matched filter 866 is matched to the signature sequence ‘1’which may be transmitted to provide for example an early warningmessage. The outputs from the first and second matched filters 864, 866are fed to first and second inputs of a comparator 868 which outputs anindication as to which of the two signature sequences was better matchedto the received signal. Depending upon. whether the degree of the bettermatch exceeds a given threshold a selector 870 then initiates furtherprocessing of the input data to extract more information about theemergency in unit 872 or terminates at 874. If the preamble symbol iscarrying the signature sequence ‘0’ indicating ‘normal operation’ thenno further processing of the signal for emergency extraction purposes isrequired. However if the EWS sequence is detected then the furtheremergency processing is in general done by the processor 872.

According to the present technique it will be appreciated that becausethe signature sequence is designed to be detected first, with thedetection of the preamble OFDM symbol in a frame, at lower signal tonoise ratios than the payload data, early warning signalling by themethod described above can provide an early warning which is more widelydetectable than the payload data. Furthermore, because the EWS messagecan be detected independently of the payload data, a receiver can beconfigured to detect the EWS message even in a standby state orpowered-off state by providing a small amount of power to only the partof the receiver (described above) which is configured to detect the EWSmessage.

For the example in which more than one (N>1) possible messages may beconveyed, the message sequence marched filter 864 can be adapted asshown in FIG. 25 to include a bank of matched fitters 864.1, 864.2,864.3 etc. For the example shown in FIG. 25, a matched filter 864.1,864.2, 864.3 is provided for each of the possible N+1 signaturesequences corresponding to message 0 (‘normal operation), MESSAGE1,MESSAGE2, MESSAGE3, MESSAGE4, to MESSAGE N although it will beappreciated that this is a functional description and a softwarearrangement could be provided in which a matched filter is adapted tohave a different impulse responses for each of the possible signaturesequences. The message processor 872 receives the outputs from therespective matched filters 864.1, 864.2, 864.3 via the comparator 868and the selector 870 and then extracts the appropriate message from thereceived signal according to which of the matched filters produces thehighest output relatively. The output is however compared with athreshold to determine that the message was transmitted to avoid a falsealarm due to the presence of noise. The message can therefore bedetected by identifying one of the possible sequences of the set ofsignature sequences. Thus, by identifying the message sequence from apossible set of sequences the information conveyed by the message isidentified. In one example the message sequence represents secondsignalling data, which may represent layer one signalling data and somay be fed to the preamble detector and decoder 430 for detecting andrecovering the payload data.

According to one example embodiment, the signalling data may be used toidentify a type of constellation which is used for carrying the layerone signalling in the signalling OFDM symbol. Thus, the secondsignalling data carried by the message sequence can represent amodulation scheme, for example, BPSK, QPSK, 16 QAM, or 64 QAM, which maybe represented by different possible sequences of the message sequence.The modulation scheme therefore represents the way in which data hasbeen modulated onto the signalling OFDM symbol. Thus, having detectedthe synchronisation sequence within the received signal to identify thesignalling frame, the message processor 872 is used to detect themessage sequence, the detected message sequence from for example fourpossible sequences each representing a different modulation schemetherefore provides the modulation scheme with which data is modulatedonto the signalling OFDM symbol. Therefore, the message processor 872feeds an output signal to the preamble detector and decoder 430 which isarranged to demodulate the data from the sub-carriers of the signallingOFDM symbol to thereby recover the signalling data which may representlayer one data of the preamble OFDM symbol.

For the example in which the message sequence is used to provide userlevel information such as an early warning message for a public safelybroadcast, then the receiver could be arranged to provide power to thepreamble detector and decoder 430 even in a powered off state or standbystate so that the preamble detector and decoder 430 can be arranged tosubstantially continuously monitor the signalling messages. In someexamples a battery may be used to provide power if the receiver is notconnected to a mains electricity supply. In some examples wherenecessary the matched filter 502 may also be provided with power whenthe receiver is not in a powered on state so that the message sequencecan be detected, although in other examples the preamble detector anddecoder 430 may be configured to provide all necessary functionality todetect the message sequence and so may only need to be poweredsubstantially continuously.

The following numbered clauses provide further example aspects andfeatures of the present technique:

1. A transmitter for transmitting payload data using OrthogonalFrequency Division Multiplexed (OFDM) symbols, the transmittercomprising

a frame builder configured to receive the payload data to be transmittedand to receive first signalling data for use in detecting and recoveringthe payload data at a receiver, and to form the payload data with thefirst signalling data into frames for transmission,

a modulator configured to modulate a first OFDM symbol with the firstsignalling data and to modulate one or more second OFDM symbols with thepayload data,

a signature sequence processor for generating a signature sequence,

a combiner for combining the generated signature sequence with the firstOFDM symbol, and

a transmission unit for transmitting the first and second OFDM symbols,wherein the signature sequence provided by the signature sequenceprocessor is selected from one of a set of signature sequences, thesignature sequence being combined by the combiner with the first OFDMsymbol, so that a receiver can detect and recover the first signallingdata from the first OFDM symbol and the signature sequences of the setprovide message information to the receiver.

2. A transmitter according to clause 1, wherein the signature sequenceprocessor comprises a signature sequence generator for generating theselected signature sequence from the set of the signature sequences anda sequence controller for selecting the signature sequence to begenerated, wherein a first of the signature sequences is selected sothat the first OFDM symbol can be detected and the first signalling datarecovered before the one or more second OFDM symbols, and one or moreother signature sequences of the set are selected to represent adifferent message.

3. A transmitter according to clause 1 or 2, wherein the first OFDMsymbol is a first type having a different number of sub-carriers thanthe one or more second OFDM symbols of a second type.

4. A transmitter according to clause 2 or 3, wherein the signaturesequence processor is configured to provide either the firstsynchronisation sequence for a receiver to use in detecting the firstOFDM symbol before the one or more second OFDM symbols or to provide oneof the other signature sequences from the set for detecting the firstOFDM symbol and indicating a message to the receiver as one of aplurality of signature sequences, each sequence representing adifference message.

5. A transmitter according to clause 4, wherein the firstsynchronisation sequence and the each of the other message sequenceseach comprises a set of complex coefficients that are combined with thefirst OFDM symbol by adding each of the complex coefficients with acorresponding sample of the first OFDM symbol in the time domain.

6. A transmitter according to any of clauses 1 to 5, wherein the set ofcomplex coefficients are based on a sequence generated using at least afirst pseudo-random binary sequence generator configured to generate areal component of the complex coefficients, and at least a secondpseudo-random binary sequence generator separately configured togenerate the imaginary component of the complex coefficients.

7. A transmitter according to clause 6, wherein each pseudo-randombinary sequence generator is formed from an M-sequence or Gold codesequence generator.

8. A transmitter according to clause 5 where in the set of complexcoefficients of the signature sequences is generated using a constantamplitude zero autocorrelation sequence generator.

9. A transmitter according to any of clauses 1 to 8, wherein theinformation includes user level information such as a publicly broadcastearly warning or the like/

10. A method of transmitting payload data using Orthogonal FrequencyDivision Multiplexed (OFDM) symbols, the method comprising

receiving the payload data to be transmitted,

receiving first signalling data for use in detecting and recovering thepayload data to be transmitted at a receiver,

forming the payload data with the first signalling data into frames fortransmission,

modulating a first OFDM symbol with the first signalling data,

modulating one or more second OFDM symbols with the payload data,

providing a signature sequence,

combining the signature sequence with the first OFDM symbol, and

transmitting the first and second OFDM symbols, wherein the providingthe signature sequence comprises

selecting the signature sequence from one of a set of signaturesequences, the selected signature sequence being combined with the firstOFDM symbol, so that a receiver can detect and recover the firstsignalling data from the first OFDM symbol and the signature sequenceselected from the set of signature sequences represents messageinformation to the receiver.

11. A method according to clause 10, wherein the providing the signaturesequence includes selecting the signature sequence front the set to begenerated, and

generating the selected signature sequence from the set of the signaturesequences, wherein a first of the signature sequences is selected sothat the first OFDM symbol can be detected and the first signalling datarecovered before the one or more second OFDM symbols, and one or moreother signature sequences of the set are selected to represent differentmessage information.

12. A method according to clause 10 or 11, wherein the first OFDM symbolis a first type having a different number of sub-carriers than the oneor more second OFDM symbols of a second type.

13. A method according to any of clauses 11 to 13, wherein the providingthe synchronisation sequence includes

providing either the first synchronisation sequence for a receiver touse in detecting the first OFDM symbol before the one or more secondOFDM symbols, or

providing one of the other signature sequences from the set fordetecting the first OFDM symbol and indicating a message to the receiveras one of a plurality of signature sequences, each sequence representinga difference message.

14. A method according to any of clauses 10 to 13, wherein the firstsynchronisation sequence and each of the message sequences comprise aset of complex coefficients and the signature sequence is combined withthe first OFDM symbol by adding each of the complex coefficients with acorresponding one of the samples of the first OFDM symbol in the timedomain.

15. A method according to any of clauses 10 to 14, wherein the set ofcomplex coefficients are based on a sequence generated using at least afirst pseudo-random binary sequence generator configured to generate areal component of the complex coefficients, and at least a secondpseudo-random binary sequence generator separately configured togenerate the imaginary component of the complex coefficients.

16. A method according to clause 16, wherein each pseudo-random binarysequence generator is formed from an M-sequence or Gold code sequencegenerator.

17. A method according to clause 15, wherein the set of complexcoefficients of the signature or message sequences is generated using aconstant amplitude zero autocorrelation sequence generator.

18. A method according to any of clauses 10 to 17, wherein theinformation includes user level information such as a publicly broadcastemergency warning or the like.

19. A receiver for detecting and recovering payload data from a receivedsignal, the receiver comprising

a detector for detecting the received signal, the received signalcomprising the payload data with first signalling data for use indetecting and recovering the payload data, the first signalling databeing carried by a first Orthogonal Frequency Division Multiplexed,OFDM, symbol, and the payload data being carried by one or more secondOFDM symbols, and the first OFDM symbol having been combined with asignature sequence,

a synchronisation detector for comprising a matched filter having animpulse response which has been matched to the signature sequence withthe effect that an output of the matched filter generates a signalrepresenting a correlation of the signature sequence with the receivedsignal, and

a demodulator for recovering the first signalling data from the firstOFDM symbol for recovering the payload data from the second OFDMsymbols, wherein the signature sequence comprises one of a predeterminedset of synchronisation sequences, which includes a first synchronisationsequence one or more second message sequences, the signature sequencehaving been selected and combined with the first OFDM symbol, the firstsynchronisation sequence being provided for a receiver to detect and torecover the first signalling data from the first OFDM symbol and the oneor more second message sequences representing message information to thereceiver, and the receiver including

a message detector for detecting and recovering the message informationby identifying the second message sequence from amongst the set ofsecond message sequences.

20. A receiver according to clause 19, wherein the message detectorincludes a second matched filter having an impulse response which hasbeen matched to the differentially encoded designated message sequence,the message detector being configured to detect the presence of thesecond message sequence from processing the output of the second matchedfilter thereby decoding the message information.

21. A receiver according to clause 19 or 20, wherein the first OFDMsymbol is a first type having a different number of sub-carriers thanthe one or more second OFDM symbols of a second type.

22. A receiver according to clause 19, 20 or 21, wherein the firstsynchronisation sequence and the second message sequence of thesignature sequence each comprise a set of complex coefficients and thesignature sequence is combined with the first OFDM symbol by adding eachof the complex coefficients with a corresponding one of the samples ofthe first OFDM symbol in the time domain.

23. A receiver according to any of clauses 19 to 22, wherein the set ofcomplex coefficients of the signature sequence is based on a sequencegenerated using at least a first pseudo-random binary sequence generatorconfigured to generate a real component of the complex coefficients, andat least a second pseudo-random binary sequence generator separatelyconfigured to generate the imaginary component of the complexcoefficients.

24. A receiver according to clause 23, wherein each pseudo-random binarysequence generator is formed from an M-sequence or Gold code sequencegenerator.

25. A receiver according to clause 23, wherein the set of complexcoefficients of the signature or message sequences is generated using aconstant amplitude zero autocorrelation sequence generator.

26. A receiver according to any of clauses 19 to 25, comprising a powersupply and a controller, wherein the controller is configured incombination with the power supply to supply power to the signallingdecoder, when the receiver is in a powered off or standby state whenpower is not supplied to some or all of the remaining parts of thereceiver.

27. A method of detecting and recovering payload data from a receivedsignal, the method comprising

detecting the received signal, the received signal comprising timedivided frames including the payload data with first signalling data foruse in detecting and recovering the payload data, the first signallingdata being carried by a first Orthogonal Frequency Division Multiplexed,OFDM, symbol, and the payload data being carried by one or more secondOFDM symbols, and the first OFDM symbol having been combined with asignature sequence,

filtering the received signal with a matched filter having an impulseresponse which has been matched to the signature sequence with theeffect that an output of the matched filter generates a signalrepresenting a correlation of the signature sequence with the receivedsignal,

detecting the first OFDM symbol from the output signal of the matchedfilter, and

demodulating the first OFDM symbol to recover the first signalling datafrom the first OFDM symbol for recovering the payload data from thesecond OFDM symbol, wherein the signature sequence comprises one of apredetermined set of synchronisation sequences, which includes a firstsynchronisation sequence one or more second message sequences, thesignature sequence having been selected and combined with the first OFDMsymbol, the first synchronisation sequence being provided for a receiverto detect and to recover the first signalling data from the first OFDMsymbol and the one or more second message sequences representing messageinformation to the receiver, and the method including

detecting and recovering the message information by identifying thesecond message sequence.

28. A method according to clause 27, the method comprising

detecting the message information using additional matched filters eachhaving an impulse response which has been matched to each of the set ofsecond message sequences each differentially processed therebycorrelating each of the second message sequences with the received firstOFDM symbol, and

detecting the message information from a highest correlation output fromeach of the matched filters in correspondence to the sequences in thepredetermined set of sequences.

29. A method according to clause 27 or 28, wherein the first OFDM symbolis a first type having a different number of sub-carriers than the oneor more second OFDM symbols of a second type.

30. A method according to any of clauses 27 to 29, wherein the firstsynchronisation sequence and the second message sequence of thesignature sequence each comprise a set of complex coefficients and thesignature sequence having been combined with the first OFDM symbol byadding each of the complex coefficients with a corresponding one of thesamples of the first OFDM symbol in the time domain.

31. A method according to any of clauses 27 to 30, wherein the firstsynchronisation sequence and the second message sequence of thesignature sequence each comprise a set of complex coefficients and thesignature sequence having been combined with the first OFDM symbol byadding each of the complex coefficients with a corresponding one of thesamples of the first OFDM symbol in the frequency domain.

32. A method according to clauses 30 or 31, wherein the the firstsynchronisation sequence comprises the set of complex coefficients ofthe signature sequence generated using at least a first pseudo-randombinary sequence generator configured to generate a real component of thecomplex coefficients, and at least a second pseudo-random binarysequence generator separately configured to generate the imaginarycomponent of the complex coefficients.

33. A method according to clause 32, wherein each pseudo-random binarysequence generator is formed from an M-sequence or Gold code sequencegenerator.

34. A method according to clause 32, wherein the set of complexcoefficients of the signature or message sequences is generated using aconstant amplitude zero autocorrelation sequence generator

35. A method according to any of clauses 27 to 34, comprising

supplying power to the signalling decoder, when the receiver is in apowered off or standby state when power is not supplied to some or allof the remaining parts of the receiver.

Various further aspects and features of the present disclosure aredefined in the appended claims. Further example aspects and features ofthe present disclosure are defined in the appended claims. Variouscombinations of features may be made of the features and method stepsdefined in the dependent claims other than the specific combinations setout in the attached claim dependency. Thus the claim dependencies shouldnot be taken as limiting.

1. A transmitter for transmitting payload data using OrthogonalFrequency Division Multiplexed (OFDM) symbols, the transmittercomprising a frame builder configured to receive the payload data to betransmitted and to receive first signalling data for use in detectingand recovering the payload data at a receiver, and to form the payloaddata with the first signalling data into frames for transmission, amodulator configured to modulate a first OFDM symbol with the firstsignalling data and to modulate one or more second OFDM symbols with thepayload data, a signature sequence processor for providing a signaturesequence, a combiner configured to combine the signature sequence withthe first OFDM symbol, and transmission circuitry configured to transmitthe first and second OFDM symbols, wherein the signature sequenceprovided by the signature sequence processor is selected from one of aset of signature sequences, the signature sequence being combined by thecombiner with the first OFDM symbol, so that a receiver can detect andrecover the first signalling data from the first OFDM symbol and thesignature sequences of the set provide message information to thereceiver.